There are numerous means for identifying malignant colorectal lesions. However, no satisfactory probes are presently available to assess the malignant risk of premalignant neoplastic lesions, and methods currently used to detect abnormal proliferation in normal-appearing, preneoplastic epithelium require freshly obtained tissue, incubation in organ culture and autoradiography with a 2-4 week development period.
The use of lectin-based diagnostic testing circumvents the requirement for fresh tissue specimens. For example, the lectin PNA (peanut agglutinin) binds to and can be used to identify premalignant lesions in previously obtained formalin-fixed biopsy specimens. A series of synthetically-modified oligosaccharides was used to determine the specific sites of interaction between PNA and the T-antigen as reported in Rinderle, Stephen J., et al., (1989) J.Biol. Chem., 264:16123-131. Experiments have demonstrated that PNA would bind T-antigen even if the C-6 hydroxyl group of N-acetylgalactosamine (GalNAc) was substituted.
The PNA lectin has been an important probe used by histochemists interested in colorectal neoplasia. See Koeppe, S. J., and Rupnow, J. H. (1988) J. Food Sci. 53, 1412-1417 and Springer, G. F., Desai, P. R., and Banatwala, I. (1975) J. Natl. Cancer Inst. 54, 335-339. PNA does not bind readily to mucins found in the normal colon, but binds to mucin secreted by colon cancers, reported in Koeppe and Kaifu, R., Plantefaber, L. C., and Goldstein, I. J. (1985) Carbohydr. Res. 140, 37-49. In addition, PNA binds to glycoconjugates present in a variety of premalignant lesions in the colon. Cooper has reported that the treatment of histological sections of human colonic tissue with neuraminidase resulted in the "unmasking" of cryptic T-antigens to which PNA is subsequently bound. However, this has not been the experience of all investigators, see Koeppe above.
Moreover, a series of oligosaccharides isolated from the mucin of normal human colonic tissues revealed no evidence of the T-antigen or its sialylated variants according to Shibata, S., Peters, B., Roberts, D. D., Goldstein, I. J., and Liotta, L. A. (1982) FEBS Lett. 142, 194-198 and Distler, J. J., and Jourdian, G. W. (1973) Biol. Chem. 248, 6772-6780.
Thus, it is not clear whether a variant form of T-antigen may be synthesized in normal colonic tissues. PNA, however, binds these lesions at a relatively late stage of neoplastic development, and is thus not useful in detecting early stage preneoplastic abnormalities. The usefulness of other lectins such as BPA (Bauhinea purpurea agglutinin) and RCA, (Ricinis communis agglutinin) is also limited in that their binding specificities are relatively broad and impede accurate interpretation of binding results.
The Thomsen-Friedenreich antigen (T-antigen) has been proposed as a specific carcinoma marker. See Springer, J. National Cancer Inst. and Springer, G. F. (1984) Science 224, 1198-1206. Thus, the development of well-defined, T-specific probes would lead to increased understanding and detection of certain types of cancer. Lectins once identified and isolated, remain particularly attractive as histochemical probes because reliable and uniform preparations of agglutinin can be easily generated and binding specificity can be documented with a high degree of precision. Thus, lectins which can detect early stage proliferation abnormalities in colorectal epithelium would be advantageous in screening normal-appearing preneoplastic epithelium to identify those at risk for colorectal cancer, and for diagnosing premalignant and malignant neoplastic lesions.